Electroplating of metals



Filed NOV. 20, 1961 lo FEED u on PUMP ELEOTROLYTE FEED L UOR PUMP iigi'i United States Patent Oflfi'ce 3,072,545 Patented Jan. 8, 1963 3,072,545 ELECTROPLATING F METALS Walt-er Juda and Thomas A. Kirkham, Lexington, Mass, assignors to Ionics, Incorporated, Cambridge, Mass, a corporation of Massachusetts Filed Nov. 20, 1961, Ser. No. 153,466 Claims. (Cl. 204-412) This invention relates to the electroplating of metals from solutions of metal salts wherein the metal is oxidizable. More particularly, the invention relates to the electroplating of iron from ferrous solutions thereof in which the oxidation of'ferrous ion to ferric ion at the anode is prevented or minimized. The invention is also directed to the electrolytic cell for effecting the process. More specifically, the invention relates to the electroplating of iron from spent pickling liquor to regenerate the same for further use. While the invention is hereinafter specifically described with respect to the plating out of the ferrous ion in a ferrous sulfate-sulfuric acid solution, the same principle can be used in ferrous solutions of other acids as well as with other metallic ions, such as tin, lead, etc., subject to electroplating at the cathode, and at the same time subject to undesirable oxidation at the anode. This application is a continuation-in-part of applicants co-pending case, Serial No. 8,270, filed February 12, 1960, now abandoned.

The use of two-compartment cells in which catholyte and anolyte are separated by means of a porous diaphragm is well known for plating of iron, especially, for example, for plating of iron from waste pickle liquor for steel. While flowing a ferrous sulfate liquor, for example, into the cathode compartment through the diaphragm and out of the anode compartment, such a two-compartment cell results in satisfactory plating of part of the iron at the cathode, but does not prevent the substantial oxidation of the remainder of the dissolved ferrous ions to ferric ions at the anode.

It is an object of this invention to provide an electrolytic process in which oxidizable metal ions (such as ferrous ions) are efficiently plated out of a solution thereof at a cathode under conditions preventing or minimizing the oxidation of ferrous ion at the anode. It is a further object of the invention to provide an electroplating cell suitable for this objective. An additional object of this invention is the recovery of the metal from the solution of halide salts thereof without the evolution of halogen gases at the anode of the electrolytic cell.

In general, the simplest embodiment of this invention is a cell unit comprising a porous diaphragm, for example, an asbestos diaphragm, an ion-exchange membrane and preferably a cation-exchange membrane, an anode, means I of flowing electrolyte solution into the compartment defined by the cathode and the diaphragm, through the diaphragm and out of the compartment defined by the diaphragm and the ion-exchange membrane, and means of introducing an electrolyte into the compartment defined by the ion-exchange membrane and the anode. The general process of this invention comprises the steps of flowing by pressure means an aqueous solution of a ferrous salt into the cathode compartment defined by the cathode and the porous diaphragm, and flowing this ferrous solution through the diaphragm, out of the cell from the compartment defined by the porous diaphragm and membrane. It further comprises the steps of introducing an electrolyte, for example an acid such as sulfuric acid, into the compartment defined by the ion-exchange membrane and the anode; finally, it comprises the step of applying D.C. energy to this cell. The solution fed as catholyte to the cell is preferably rich in ferrous ion. The flow of catholyte through the porous diaphragm is adjusted so as to wash back the hydrogen ions introduced into the flowing electrolyte from the anolyte across the cation-exchange membrane. In this manner, the efiiciency of iron deposited at the cathode is maintained at a high level since the hydraulically flowing catholyte washes the interfering hydrogen ion back through the diaphragm and away from the cathode. Similarly, by the placement of the cation membrane barrier as a buffer against the sulfuric acid, the ferrous ion is kept away from the anode, thereby preventing its oxidation to the ferric ion. Because of the very high mobility of the hydrogen ion moving from the anolyte across the cation-exchange membrane into the flowing catholyte, the membrane efficiency is at a high level, for example, in excess of 70 percent. Similarly, because of the high ferrous concentration with respect to hydrogen ions near the cathode, the plating efficiency of iron is equally kept at a high level, again, for example, in excess of 70 percent efficiency.

One important application of this invention is the regeneration of waste pickle liquor for steel. A typical aqueous pickle liquor consists of a solution of sulfuric acid and ferrous sulfate. In order to obtain high current efficiencies for plating, an overall flow sheet involves, for example, evaporating the spent pickle liquor for the purposes of concentrating free sulfuric acid and crystallizing therefrom ferrous sulfate. The free concentrated sulfuric acid is directly suitable for reuse. The ferrous sulfate crystals are then dissolved in water (with some excess sulfuric acid clinging to the crystals) and a slightly acid ferrous sulfate solution is introduced as catholyte into the cell of the drawing, then through the porous diaphragm and out from the compartment defined by the diaphragm and the ion-exchange membrane. In this Way the deposit of iron occurs at high current efficiencies.

FIGURE 1 is a diagrammatic representation of a single electrolytic cell unit of the present invention, wherein three compartments of cell 1 are defined by spaced ionexchange membrane 2 and porous diaphragm 3. The three compartments are indicated as 4, 5 and 6, representing the cathode compartment containing cathode 7, the

center compartment, and anode compartment containing the anode 8. The center compartment is defined by spaced ion-exchange membrane 2 adjacent the anode compartment 6, while the porous diaphragm 3 is adjacent-the cathode compartment 4. inlet means for the feed solution is provided for at 9 by way of pressure means 10 (such as a pump), an outlet means for the reacted solution from center compartment 5 is indicated at Ill. Inlet and outlet means for passing an electrolyte solution through the anode compartment 6 is indicated at 13 and 12, respectively. Means for providing a DC. potential transversely acrossthe three compartments and the membrane and dia phragm is provided for through 29 and 30 from an outside source (not shown).

The operation of the cell of FIGURE 1 may be illustrated, for example, in the electroplating of iron in the regeneration of pickle liquor; Spent pickle liquor containing ferrous sulfate and some sulfuric acid, is introduced into the cathode chamber 4 as at 9 under a pressure sufficient to flow said liquor through the porous diaphragm 3 into and out of the center compartment 5. Dilute sulfuric acid solution is passed through the anode compartment 6. Upon the impression of sufficient external voltage, iron is plated out at the cathode and oxygen is evolved at the anode. Hydrogen ions are transferred from the anolyte through the cation-exchange membrane 2 into the chloric acid or even hydrofluoric acid are useful in this cell as pickling liquors since the hydrochloric and hydron fluoric acid can be recovered without the evolution of the halogen gas at the anode. In this event, the chloride or fluoride pickle liquor are used as the catholyte from which iron, for example, is plated out, whereas an acid such as sulfuric or phosphoric acid, or the like, is used as the anolyte, preventing the evolution of a halogen gas.

FIGURE 2 is a diagrammatic representation of another embodiment of this invention that of a multiple unit electrolytic cell wherein the use of common electrodes are used advantageously in place of single electrodes. For the purposes of this invention, the term common electrodes as used herein is defined as an electrode in which both sides of said electrode are actively taking part in the electrolytic process, and the term single electrode refers to an electrode wherein only one side is used in the electrolytic process. In FIGURE 2 there is shown a plurality of repeating units of single electrolytic cells arranged adjacent to each other wherein each cathode is common to two individual units, such cathode 7, in conjunction with adjacent spaced diaphragms 3, defining the cathode chambers 4; and the anodes being arranged common to two units in conjunction with the oppositely spaced selective ion-exchange membranes 2 defining the anode chambers 6. The anodes 50 at the terminal ends of the multiple cell unit are employed as single electrodes, said multiple cell however may obviously have terminating electrodes comprising cathodes, or a cathode and an anode. The center compartments 5 are defined by spaced cation-selective ion-exchange membranes 2 and adjacently spaced porous diaphragms 5. Inlet means for the feed solution to the cathode compartments is provided for by manifold inlet 9 and by way of pressure means 10, and outlet means for the reacted solution from center compartments 5 is indicated by manifold 11. Inlet and outlet means for passing an electrolyte solution through the anode compartments 6 are indicated at manifolds 13 and 12, respectively. Means for providing a DC. potential transversely across each electrolytic cell unit is provided by leads 20 and 353' from an outside source of electric current (not shown), all cathodes 7 being connected electrically in parallel by means of busbar 22 and all anodes similarly connected in parallel by means of busbar 32.

The multiple unit electrolytic cell provides a more convenient and commercial iethod of electroplating out metals in accordance with the process of this invention as will be hereinafter described. Such embodiment provides a method of electroplating which satisfactorily reduces or avoids many operational difficulties encountered in the use of a single cell unit. Other advantages are lower pumping and construction cost and compactness in fabrication. The use of common electrodes in which both sides are actively used in the electrolytic process effects a substantial saving in labor and cost of electrode material.

It has been found that in the use of an electrolytic cell employing a non-common or single cathode, operational difficulties are often encountered where electro-deposition of the reduced metal occurs on only one side of the cathode. During the electrochemical reduction of the metal ion, for example iron in the regeneration of pickle liquor, the metal is continuously deposited as crystals on said cathode surface forming a hard, adherent mass of recoverable iron. Said electrolytic deposit does not collect in a uniform thickness, but tends to form trees or accretions accompanied by internal physical stresses within said deposited layer resulting in a bending or warping of the cathode sheet. The thinner the initial cathode sheets, the less deposited material necessary to develop distortion, and in general distortion will occur before the cathode has doubled itself in thickness from the deposited metal. The use of a common cathode as used in the process of this invention, whereby metal is deposited in substantially equal amounts on both sides of the cathode, allows a greater buildup of the metal mass without cathode distortion, since the physical stresses occurring during the deposition of the metals are equalized when both sides i of the cathode are used as plating surfaces. It is evident that prevention of cathode distortion allows greater ease in removal of the cathode plates from the electrolytic cell and the reuse of said plates after removal of the deposited metal layer.

Another difliculty often encountered in one-sided cathodic plating is chemical corrosion of the unused side of the cathode. This is especially pronounced when the cathode material is of a mild steel composition and where the feed solution to the cathode compartment is of an acid nature, for example, a Fe SO -sulfuric acid pickle liquor solution. When the feed solution of pickle liquor is in the nature of a halogen acid, as for example a FeCl hydrochloric acid mixture, attack on the mild steel cathode is even more severe. The use of a common cathode in accordance with this invention will minimize or prevent cathode chemical attack since both surfaces of said cathode will be cathodically protected by the electrodeposited layer of reduced metal.

The operation of the multiple electrolytic cell of FIG- URE 2 may be illustrated, for example in the electroplating of iron in the regeneration of spent pickle liquor, for example liquor containing ferrous chloride and some hydrochloric acid. The spent pickle liquor is directed into the cathode chambers 4 by means of conduit 9 under sufficient pressure by way of pressure means 10 to force said liquor through the liquid permeable porous diaphragm 3 into and out of center compartments 5 and into effiuent conduit 11. An electrolyte of dilute sulfuric acid is introduced into and removed from anode compartments 6 by means of conduits 13 and 12 respectively. Upon the impression of sufficient external voltage across each unit, iron is plated out on each side of the common cathodes 7 and oxygen is evolved at the anodes 8.

The following examples are illustrative of the practice of this invention and are not intended to be limiting:

Example 1 An electrolytic cell of the design of FIGURE 1 is constructed containing a cathode made of mild steel which has previously been pickled with nitric acid (to provide a surface to which the iron plated from the solution adheres). The diaphragm is an asbestos paper maintained in place between two screens of saran. The cationexchange membrane is a sulfonated copolymer of styrene and divinyl benzene reinforced with an embedded material made in accordance with U.S. Patent No. 2,730,768. Other cation-exchange membranes, as for example described in U.S. Patent Nos. 2,702,272, 2,731,411 and 2,731,408, may also be used. The anode is a sheet of lead. The catholyte fed to this cell is a liquor which is 3 N in ferrous sulfate and .2 N in sulfuric acid. The anolyte is 31 percent sulfuric acid solution. The ferrous sulfate containing liquor is fed to this cell at a rate of 1 milliliter per minute per square inch of electrode area. The temperature of the cell is kept at 60 C. A cathode is spaced from the diaphragm at approximately A of an inch. Similarly, the diaphragm and ion-exchange membranes were also spaced at about Mr of an inch. The distance between the ion-exchange membrane and the anode was maintained at about /8 of an inch. Under those conditions, efiicient plating was carried out under a voltage of 4-4.5 volts with a current density of 60-70 milliamps. per square centimeter. The pH of the catholyte fed to the cell was maintained above 1.5 by adjustment with ammonia. The iron plating as well as acid producing efiiciency was about percent.

Example 2 The procedure of Example 1 was carried out with the feed pickle liquor to the cathode chamber comprising about 3 N ferrous chloride and about 0.2 N hydrochloric acid. A 30 percent solution of sulfuric acid was fed into the anode chamber. With all the other operating conditions being maintained as in Example 1, the iron plating at the cathode was very cflicient in conjunction with acid formation, and substantially no chlorine gas formation at the anode.

Example 3 The procedure of Example 1 was carried out using a multiple cell, such as is shown in FIGURE 2, comprising four single repeating cell units, employing common electrodes in the body of the cell. At the extreme ends of the multiple cell, single anodes were used. The cathodes were composed of sheets of mild steel thick and plating Was carried out on both surfaces of said common cathodes. The electroplating continued until the deposited layer of reduced iron measured A2. thick on both sides of the cathodes with no notable evidence of cathode distortion or corrosion. Efiicient plating was carried out under a voltage of 4-4.2 volts per single cell unit with a current density of 65-70 milliamps. per square centimeter of cathode area. An efliciency of 8085% was obtained.

What is claimed is:

1. The method of electroplating metals from solutions of metal salts wherein the metal is oxidizable comprising, passing a feed of the metal salt solution into the cathode compartment of a three-compartment electrolytic cell having a cathode compartment separated from a center compartment by a fluid-permeable porous diaphragm and the center compartment separated from the anode compartment by a cation-selective ion-exchange membrane, maintaining a greater pressure in the cathode compartment than in the center compartment to cause said feed solution to flow from the cathode compartment through the porous diaphragm into and out of the center compartment, introducing an acid producing electrolyte into the anode compartment, passing a direct current transversely through said compartments, diaphragm, and membrane to cause plating of the metal on the cathode with minimum oxidation of any remaining metal ions at the anode, and removing the metal depleted solution from the center compartment.

2. The method of regenerating spent pickle liquor in accordance with claim 1, wherein the feed liquor introduced into the cathode compartment is a solution of ferrous sulfate in dilute sulfuric acid and the electrolyte introduced into the anode compartment is a sulfuric acid solution, and withdrawing the regenerated pickle liquor from the center compartment.

3. The method of claim 2, wherein the spent pickle feed liquor is a solution of ferrous halide, the electrolyte introduced into the anode compartment is a solution of a non-halogen acid, and wherein the evolution of a halogen gas at said anode is substantially eliminated.

4. The method of claim 3, wherein the halide and halogen are chloride and chlorine, respectively.

5. The method of electroplating metals from solutions of metal salts wherein the metal is oxidizable, comprising passing a feed of the metal salt solution into the cathode compartments of a multiple unit of repeating threecompartment electrolytic cells, each cell having a cathode compartment separated from the center compartment by a fluid-permeable porous diaphragm, the center compartment separated from the anode compartment by a cationselective ion-exchange membrane, cathodes and anodes located in said respective compartments being common to the adjacent repeating cells of saidmultiple unit,

maintaining a greater pressure in the cathode compartments than in the center compartments to cause said feed solution to flow from the cathode compartments through the porous diaphragms into and out of the center compartments, introducing an acid producing electrolyte into the anode compartments, passing a direct current transversely through said compartments, diaphragms, and membranes to cause plating of the metal on both sides of the common cathodes with minimum oxidation of any remaining metal ions at the common anodes, and removing the metal depleted solution from the center compartments.

6. The method of regenerating spent pickle liquor in accordance with claim 5, wherein the feed liquor introduced into the cathode compartments is a solution of ferrous sulfate in dilute sulfuric acid and the electrolyte introduced into the anode compartments is a dilute sulfuric acid solution, and withdrawing the regenerated pickle liquor from the center compartments.

7. A three-compartment electrolytic cell for electroplating metals from solutions of metal salts wherein the metal is oxidizable, comprising: a cathode compartment containing a cathode adapted for electroplating metals thereon, a center compartment, an anode compartment containing an anode therein, said cathode compartment being separated from the center compartment by a fluid-permeable porous diaphragm, the center compartment being separated from the anode compartment by a cation-selective ion-exchange membrane, an inlet only in the cathode compartment for a feed liquor, an inlet and outlet in the anode compartment for passing an electrolyte therethrough, means for forcing feed liquor into said cathode compartment and through said porous diaphragm into said center compartment, an outlet only in the center compartment for Withdrawing the reacted catholyte, and means for passing a direct current transversely through said compartments, membrane and diaphragm.

8. The cell of claim 7 wherein the porous diaphragm is an asbestos sheet and the cathode is mild steel.

9. A multiple electrolytic unit comprising a plurality of repeating cells of claim 7 wherein said cathodes and anodes are common to the adjacent repeating cell of said multiple unit, and including manifold means connected to the inlets of all the cathode compartments, separate manifold means connected to the inlets and outlets of the anode compartments, and manifold means connected to all the outlets of the center compartments for withdrawing all the reacted catholytes therefrom.

=10. The multiple unit of claim 9 wherein the porous diaphragms are asbestos sheet materials and the cathodes are mild steel.

References Cited in the file of this patent UNITED STATES PATENTS 2,694,680 Katz et al. Nov. 16, 1954 2,810,686 Bodamer et al Oct. 22, 1957 2,865,823 Harris et al Dec. 23, 1958 2,872,407 Kollsman Feb. 3, 1959 3,017,338 Butler et a1. Jan. 16, 1962 FOREIGN PATENTS 7 570,265 Canada Feb. 10, 1959 

1. THE METHOD OF ELECTROPLATING METALS FROM SOLUTIONS OF METAL SALTS WHEREIN THE METAL IS OXIDIZABLE COMPRISING PASSING A FEED OF THE METAL SALT SOLUTION INTO THE CATHODE COMPARTMENT OF A THREE-COMPARTMENT ELECTROLYTIC CELL HAVING A CATHODE COMPARTMENT SEPARATED FROM A CENTER COMPARTMENT BY A FLUID-PERMEABLE POROUS DIAPHRAGM AND THE CENTER COMPARTMENT SEPARATED FROM THE ANODE COMPARTMENT BY A CATION-SELECTIVE ION-EXCHANGE MEMBRANE, MAINTAINING A GREATER PRESSURE IN THE CATHODE COMPARTMENT THAN IN THE CENTER COMPARTMENT TO CAUSE SAID FEED SOLUTION TO FLOW FROM THE CATHODE COMPARTMENT THROUGH THE POROUS DIAPHRAGM INTO AND OUT OF THE CENTER COMPARTMENT, INTRODUCING AN ACID PRODUCING ELECTROLYTE INTO THE ANODE COMPARTMENT, PASSING A DIRECT CURRENT TRANSVERSELY THROUGH SAID COMPARTMENTS, DIAPHRAGM, AND MEMBRANE TO CAUSE PLATING OF THE METAL ON THE CATHODE WITH MINIMUM OXIDATION OF ANY REMAINING METAL IONS AT THE ANODE, AND REMOVING THE METAL DEPLETED SOLUTION FROM THE CENTER COMPARTMENT.
 7. A THREE-COMPARTMENT ELECTROLYTIC CELL FOR ELECTROPLATING METALS FROM SOLUTIONS OF METAL SALTS WHEREIN THE METAL IS OXIDIZABLE, COMPRISING: A CATHODE COMPARTMENT CONTAINING A CATHODE ADAPTED FOR ELECTROPLATING METALS THEREON, A CENTER COMPARTMENT, AN ANODE COMPARTMENT CONTAINING AN ANODE THEREIN, SAID CATHODE COMPARTMENT BEING SEPARATED FROM THE CENTER COMPARTMENT BY A FLUID-PERMEABLE POROUS DIAPHRAGM, THE CENTER COMPARTMENT BEING SEPARATED FROM THE ANODE COMPARTMENT BY A CATION-SELECTIVE ION-EXCHANGE MEMBRANE, AN INLET ONLY IN THE CATHODE COMPARTMENT FOR A FEED LIQUOR, AN INLET AND OUTLET IN THE ANODE COMPARTMENT FOR PASSING AN ELECTROLYTE THERETHROUGHM MEANS FOR FORCING FEED LIQUOR INTO SAID CATHODE COMPARTMENT AND TROUGH SAID POROUS DIAPHRAGM INTO SAID CENTER COMPARTMENT, AN OUTLET ONLY IN THE CENTER COMPARTMENT FOR WITHDRAWING THE REACTED CATHOLYTE, AND MEANS FOR PASSING A DIRECT CURRENT TRANSVERSELY THROUGH SAID COMPARTMENTS, MEMBRANE ND DIAPHRAGM. 